The present invention relates to a liquid crystal color display device comprising:
(1) a liquid crystal cell comprising a pair of substrates, a liquid crystal layer positioned between the substrates and a transparent electrodes on the substrates to apply voltage to the layer;
(2) a pair of polarizers positioned on opposite sides of the cell; and
(3) a combination of a color filter film and an optical phase plate comprising a liquid crystal polymer composition, having a polyester as a main constituent, which is thermotropic and has a glass phase below the temperature range of liquid crystalline phase positioned between the liquid crystal layer and at least one of the polarizers.
A mode of display of a liquid crystal (hereinafter referred to as "LC") so far mainly used, is an one called Twisted Nematic (hereinafter referred to as "TN") type which has LC molecular configuration twisted about 90.degree. between a pair of substrates and utilizes a rotation of polarization plane by the configuration and a cancellation of the rotation by a voltage application. Although this display device has advantages that as this device is for a white and black display, it has an excellent shutter effect and when provided with color filter on each of color elements, it is relatively easy to use this device as a multicolor display device, it has a disadvantage that its highly multiplexing drive is difficult because of its bad threshold characteristics on a voltage-transmittance relationship. Therefore, on a display with large information content, there exist several problems such as a low contrast and a narrow viewing angle.
To solve these problems by improving a steepness of a relationship between voltage and transmittance, an idea was proposed to utilize a birefringent effect of LC by making a gap between a polarizing axis of the polarizer and an aligned direction of LC molecule, of which twist angle is larger than TN. A mode improved by the idea are called super twisted birefringence effect (hereinafter referred to as "SBE") or super twisted nematic (hereinafter referred to as "STN") mode.
Although the mode had an excellent features that its contrast was less lowered and its viewing angle was wide even at a highly multiplexing drive because of its excellent threshold value, the mode gave a tinted display due to its birefringent effect and the colorization was very difficult.
To improve the tinted image of STN mode, a two-layer type STN mode LC display device was recently developed. This type had two LC cells having reverse twisted angle each other, one for deriving and the other for color compensation and exhibits a black and white display. Since this device had two LC cells, the device itself became thick and heavy and its productivity was bad.
Although these disadvantages were able to improve by applying a birefringent LC polymer film instead of the LC cell for compensation, which is so-called "a phase-plate type black and white display STN LC display device". However, the phase-plate type device was not able to get a sufficient contrast and further its viewing angle became narrow.
In the two-cell type device, it was recommended to use a LC polymer, having twisted molecular configuration, as a compensation plate instead of the cell for compensation. This type used a coated and aligned LC polymer film as the main constituent of the compensation plate.
Generally speaking, to use a LC material for a display device, it is necessary to align LC molecules in a certain direction. These LC molecules change their alignment under an affect of a force from outside such as electric field, magnetic field or shear stress. Accordingly, LC is utilized in a field of various electronics by utilizing changes of its optical properties accompanied with the above alignment change.
LC is largely divided into a polymer type and a low molecular weight type. Among these, LC polymer has higher viscosity in LC phase compared with low molecular weight LC (hereinafter referred to as "general type LC"). Due to the higher viscosity, LC polymer has a unique property which make it possible to align the polymer under LC phase and immobilize the alignment by cooling the polymer lower than its glass transition point. Using this unique property, several application of the polymer in an optical electronic field such as (i) an aligning film for a general type LC [Japanese Patent Application, Laid-Open (KOKAI), {hereinafter referred to as "JPA-L"} No. 61-42,618 (1986)], non-linear optical device [JPA-L, No. 62-201,419 (1987)], optical memory [JPA-L, No. 62-66,990 (1987)] and optical filter [JPA-L, No. 60-191,203 (1985)] have been tried, and to realize these trials, it is necessary to strictly control its molecular alignment in desired forms. For instance, a color compensation plate of a STN type LC display device, which is a kind of optical filters and is placed between the LC cell and the polarizer, is necessary to have a function to bring back a light elliptically polarized by passing through the LC cell to a linearly polarized light. The function can only be developed by aligning the LC polymer molecules parallel to a plane of the plate and also in one direction in uniformity and in high order.
An aligning method of general type LC has almost been established but the method aligning LC polymer has not necessarily been sufficiently established. Within a limited area only, some methods to align LC polymer even with a better order parameters than those of a general LC, not withstanding the LC is nematic, smectic or cholesteric type, are already known. However, these methods use an outside force such as electric field, magnetic field or shear stress, there are still problems that to control the alignment with a large area is impossible and although it is possible to align LC molecule parallel to a plane, it is impossible to align the polymer molecules in one direction in the plane. If LC polymer is injected into a space between a pair of substrates having received an aligning treatment, which is a usual method to obtain a general LC cell, because of high viscosity of the polymer in a LC phase, the LC polymer molecules are oriented along a flow of the polymer entering into the space and we can not have an alignment desired. Further, for a device with a large area, an injection of the LC polymer is very difficult because of its high viscosity.
In the two LC cell type device mentioned before, when a LC polymer film is used instead of the cell for compensation, since only one substrate is enough to hold LC polymer film, taking advantage of its self-standability, the device can be thinner comparing to the two-cell type LC display device so far developed and can have as good contrast as that of the two-cell type device. However, still in this case an increase of thickness and weight of the device by an additional substrate to support the LC polymer film can not be avoided.
Incidentally, when an optical phase plate is prepared by stretching conventional polymer film, it is impossible to introduce a twisted configuration in the film. Therefore, when a combination of the plate and a color filter film is used for a multi-color or a full-color display, though it can be used with certain difficulties, it is especially preferable to improve several points, for instance, narrow color-reproducible range or a dark image because of a necessity to increase a optical density of the color filter.
As a result of the present inventors' extensive studies to solve disadvantages mentioned so farand obtained an excellent LC color display device, it has been found that:
(1) an optical phase plate used in the conventional LC color display device can largely be improved by replacing it with a film of LC polymer composition, which is thermotropic and has a glass transition point,
(2) further, it is preferable to use a LC polymer composition which contains a polyester, as a main constitutent, containing an aromatic unit of specific construction as an one of constitutents; and
(3) by placing a combination of a color filter and the improved phase plate between a LC cell and at least one of the polarizers, a LC color display device being thin in thickness, light in weight, high in contrast, wide in viewing angle, high in productivity and less color dependency upon viewing angle can be obtained. Further, they have found that an alignment of LC polymer composition of the present invention should be strictly controlled.
Based upon these findings, the present inventors achieved the present invention.